Tower rebar bonding

[K7LXC@aol.com]

In a message dated 97-01-05 11:15:54 EST, you write:

>I understand wanting good bonding for the grounding, but at what cost?  From
>various sources including engineering firms, the welding of rebar materials
>weakens the rebar in the process.  Thus, appears we need to decide if we
>want to maintain structural integrity of the base and provide additional
>grounding externally, which would include grounding rods ajoining the tower.

     We've all driven by highway or building projects that use large, welded
rebar fixtures so my feeling is that it is an accepted industry practice.
 And those are just for structural purposes, they're not specifically
electrically bonded for grounding use.  My inference is that the rebar isn't
weakened enough to worry about.  If you're worried about weakening the rebar,
what about increasing the size of rebar used to compensate?
>
>The comments regarding the exploding concrete always make me ask when and
>where?  Have these situations been investigated (any documentation?) by an
>engineer and/or others in the various fields to see if an explanation might
>revel no other real ground path?  
>
     The folklore still exists about lightning exploding concrete tower
bases.  Nowadays companies like Polyphaser, etc. have done extensive research
into lightning and its effects - Polyphaser even has their own lightning
generator.  The current feeling is that the cases of exploding concrete were
due to voltage potential differences between the elements of the base
(concrete, rebar, earth).  It's those potential differences that cause
lightning damage remember.  And especially if it arced between pieces of
rebar.

>As I said, I'm not an expert in these areas, only an interested amateur that
>has read and researched to the point I'm comfortable with my installations.
>Regarding the ufer ground, I've found it commonly referenced that the
>concrete is the connection to ground, not the rebar.
>
     Of course, the concrete is in direct contact with the earth.  But an
Ufer ground utilizes the built-in rebar along with the naturally conductive
properties of the concrete to provide a viable ground point.  In the cases of
towers installed with limited ground space, sometimes the tower base Ufer
grounds present the only grounding available.  Besides, you're still trying
to keep the voltage potentials equal so that they all rise and fall at the
same rate:  Gee, that almost sounds like a definition of a ground system.

73,  Steve  K7LXC

    TOWER TECH -- professional tower supplies and services for amateurs

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